In a typical Mach-Zehnder modulator, input light from a laser is split at the input of the modulator into two waveguides, and one or both arms of the waveguide are phase modulated. When the light from the two arms is recombined, constructive interference occurs if the phase of light in the two branches are equal, and destructive interference occurs if the phase of light in the two branches is opposite. Thus by changing the relative phase of the light in the two arms using an electrical signal to alter the index of refraction of material in the waveguide, the recombined output is modulated.
FIG. 1 illustrates a typical Mach-Zender modulator (MZM) 100 as discussed above. The MZM 100 may comprise, for example, Lithium Niobate (LN). A laser 102 may produce and optical input signal 104 that enters MZM 100. The optical input signal 104 is equally split by Y-junction 106 into two waveguides 108 and 110. And the signals in each waveguide may be phase modulated. The optical signal is recombined at Y-junction 112 and output as output optical signal 114.
During phase modulation, electrodes acting on phase modulation portions 116 and 118 are used to affect the modulation of the optical signals in each waveguide 108 and 110. A voltage applied to phase modulator 116 induces a change in the index of refraction. In one state, such as when a voltage is not applied to phase modulator 118, the optical signals re-combined at Y-junction 112 are in-phase and interfere constructively. In this case, optical output signal 114 may be “high” and may be recognized as a logical “1.”
In a second state, such as when a voltage is applied to phase modulator 118, the index of refraction may change causing a phase shift between the two arms of MZM 100. When the optical signals recombine at Y-junction 112, destructive interference occurs leading to a “low” optical output signal 106 that may be interpreted as a logical “0.”
In practice it is possible to modulate the phase of only one arm of the Mach-Zehnder or to modulate both arms but in opposite directions, called “push-pull” operation. The second technique is typically faster although it requires more contacts and more power.
In any event, it would be preferred to reduce the size of the MZM and to improve overall efficiency.